Liquid crystals (LC) consist of anisotropic molecules. The average direction of the long molecular axes is called the director, d. Reorientation of the director caused by the application of an external electric field is the basis of operation of most LC devices. The basic unit of LC devices is a LC cell, which consists of two substrates with LC material sandwiched in between.
The sensitivity of a LC material to an applied electric field is determined by the dielectric anisotropy, Δ∈, and spontaneous polarization, P. P has a nonzero value only for some chiral smectic LC phases. The higher the Δ∈ and P, the lower are the operating voltage and the faster the electro-optical response of the LC device and thereby, the faster the switching time between light and dark states of the LC cell.
Nematic LC's are the most commonly used LC materials. Their electro-optical response is typically related to the square of the electric field. To increase Δ∈ and P, multi-component LC mixtures have been developed and special molecular dopants have been synthesized. This approach is extremely laborious and very expensive.
The rapid growth of portable devices incorporating liquid crystal displays has increased the demand for ever better liquid crystal materials. This has put a great demand on the design and synthesis of new LC materials. For example, the typical value of the dielectric anisotropy Δ∈ for many liquid crystals is now about 20. The typical value of the birefringence Δn for most liquid crystals is about 0.2. While liquid crystals with Δ∈˜40-50 and Δn˜0.4 have recently been reported they have yet to be achieve all of the properties required for successful commercialization. In addition, it may be desired to change the dielectric anisotropy of a liquid crystal without changing its birefringence, or vice versa. It is difficult to do this by traditional chemical synthetic methods because those two characteristics are related. Next, very often it is desired to keep the mentioned parameters but change or tailor the phase transition points of a liquid crystal to some particular applications or a temperature range. All the described changes of the parameters of existing liquid crystals require long research effort of many specialists and significant financial investments.
It is known that the sensitivity of isotropic liquids to an applied electric field can be increased by doping with ultra-fine (less than 1 micrometer (μm) size) ferroelectric particles. Ferroelectric particles are particles which have a spontaneous electric polarization that is reversible by an electric field. For example, Bachmann and Barner showed that ferroelectric BaTiO3 particles that have been finely milled in the presence of surfactant will form a stable suspension in heptane (“Stable Suspensions of Ferroelectric BaTiO3-Particles,” Solid State Communications, 68(9), 865-869 (1988)). The particles had an average radius of about 10 nm and carried a permanent dipole moment of about 2000 De. The birefringence of the suspension, which is impossible to achieve in a pure heptane matrix, was controlled by application of an electric field.
Similar effects were observed when ferromagnetic particles were imbedded in anisotropic materials. For example, it has been shown that dispersed ferromagnetic particles greatly enhance the magnetic properties of liquid crystals. At the same time, large (≧μm) particles form defects, producing large director deformations in liquid crystals. Ensembles of these particles and defects can form complex structures. High concentrations (>2-3% by weight) of sub-micron particles can create almost rigid liquid crystal suspensions.
In the parent application (Patent Publication No. US20040156008), followed by an article published in APL (Yu. Reznikov, O. Buchnev, O. Tereshchenko, V. Reshetnyak, A. Glushchenko, and J. West, “Ferroelectric nematic suspension” Applied Physics Letters, Vol. 82, No 12, p.p. 1917-1919, 2003), the inventors demonstrated that at low concentrations, liquid crystal/ferroelectric particle suspensions behave as a pure liquid crystal with no evidence of the dissolved particles other than the enhanced properties. These dilute suspensions are stable, because the nano-particles do not significantly perturb the director field in the liquid crystal, and interaction between the particles is weak. We described that doping a nematic liquid crystal with ferroelectric nanoparticles produces enhanced dielectric anisotropy and sensitivity of a nematic based suspension to the polarity of an applied electric field.
The present invention is directed toward fast and simple modification of the properties of existing liquid crystal materials by doping them with ferroelectric particles of various kinds. In addition to the dielectric anisotropy, the particles influence optical anisotropy, phase transition temperatures, and order parameter of liquid crystals.
In the present application, we report an alternative, easier, and more elegant approach than that described previously. We modify the properties of existing liquid crystals by adding inorganic particles. Particles of small enough size to share their intrinsic properties with the whole liquid crystal matrix, thereby behaving much like a molecular additive. We can tune the properties of the liquid crystals and thereby enhance the performance of a wide variety of devices. These particle composites can be also combined with polymers to create polymer dispersions, like polymer network liquid crystals, PDLC, and Stressed Liquid Crystals. The new materials are of a high importance for display industry, creation of adjustable lenses, beam steering and other light controlling devices.